This invention relates to an electrophotographic printing machine, and more particularly, concerns an improved air knife of a top vacuum corrugation feeder for such a machine.
Present high speed xerographic copy reproduction machines produce copies at a rate in excess of several thousand copies per hour, therefore, the need for a sheet feeder to feed cut copy sheets to the machine in a rapid, dependable manner has been recognized to enable full utilization of the reproduction machine's potential copy output. In particular, for many purely duplicating operations, it is desired to feed cut copy sheets at very high speeds where multiple copies are made of an original placed on the copying platen. In addition, for many high speed copying operations, a document handler to feed documents from a stack to a copy platen of the machine in a rapid dependable manner has also been reorganized to enable full utilization of the machine's potential copy output. These sheet feeders must operate flawlessly to virtually eliminate the risk of damaging the sheets and generate minimum machine shutdowns due to uncorrectable misfeeds or sheet multifeeds. It is in the initial separation of the individual sheets from the sheet stack where the greatest number of problems occur.
Since the sheets must be handled gently but positively to assure separation without damage through a number of cycles, a number of separators have been suggested such as friction rolls or belts used for fairly positive document feeding in conjunction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such as sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also been utilized.
While the friction roll-retard systems are very positive, the action of the retard member, if it acts upon the printed face can cause smearing or partial erasure of the printed material on the document. With single sided documents if the image is against the retard mechanism, it can be smeared or erased. On the other hand, if the image is against the feed belt it smears through ink transfer and offset back to the paper. However, with documents printed on both sides the problem is compounded. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
In addition, currently existing paper feeders, e.g., forward buckle, reverse buckle, corrugating roll, etc., are very sensitive to coefficients of friction of component materials and to sheet material properties as a whole.
One of the sheet feeders best known for high speed operation is the top vacuum corrugation feeder with front air knife. In this system, a vacuum plenum with a plurality of friction transport belts arranged to run over the vacuum plenum is placed at the top of a stack of sheets in a supply tray. At the front of the stack, an air knife is used to inject air into the stack to separate the top sheet from the remainder of the stack. In operation, the vacuum pulls the top sheet up and acquires it while air is injected by the air knife towards the stack to assure the separation of the top sheet from the rest of the stack. Following acquisition and seapration, the belt transport drives the sheet forward off the stack of sheets. In this configuration, separation of the next sheet cannot take place until the top sheet has cleared the stack. In this type of feeding system every operation takes place in succession or serially and therefore the feeding of subsequent sheets cannot be started until the feeding of the previous sheet has been completed. In addition, in this type of system the air knife may cause the second sheet to vibrate independent of the rest of the stack in a manner referred to as "flutter". When the second sheet is in this situation, if it touches the top sheet, it may tend to creep forward slightly with the top sheet. The air knife then may drive the second sheet against the first sheet causing a shingle or double feeding of sheets. Also, current top and bottom vacuum corrugation feeders utilize a valved vacuum feedhead, e.g., U.S. Pat. Nos. 4,269,406 and 4,451,028 which are included herein by reference. At the appropriate time during the feed cycle the valve is actuated, establishing a flow and hence a negative pressure field over the stack top or bottom if a bottom vacuum corrugation feeder is employed. This field causes the movement of the top sheet(s) to the vacuum feedhead where the sheet is then transported to the take-away rolls. Once the sheet feed edge is under control of the take-away rolls, the vacuum is shut off. The trail edge of this sheet exiting the feedhead area is the criteria for again activating the vacuum valve for the next feeding.
Other prior art feeder systems that may be relevant are as follows:
U.S. Pat. No. 2,979,329 (Cunningham) describes a sheet feeding mechanism useful for both top and bottom feeding of sheets wherein an oscillating vacuum chamber is used to acquire and transport a sheet to be fed. In addition, an ail blast is directed to the leading edge of a stack of sheets from which the sheet is to be separated and fed to assist in separating the sheets from the stack.
U.S. Pat. No. 3,424,453 (Halbert) illustrates a vacuum sheet separator feeder with an air knife wherein a plurality of feed belts with holes are transported about a vacuum plenum and pressurized air is delivered to the leading edge of the stack of sheets. This is a bottom sheet feeder.
U.S. Pat. No. 2,895,552 (Pomper et al.) illustrates a vacuum belt transport and stacking device wherein sheets which have been cut from a web are transported from the sheet supply to a sheet stacking tray. Flexible belts perforated at intervals are used to pick up the leading edge of the sheet and release the sheet over the pile for stacking.
U.S. Pat. No. 4,157,177 (Strecker) illustrates another sheet stacker wherein a first belt conveyor delivers sheets in a shingled fashion and the lower reach of a second perforated belt conveyor which is above the top of the stacking magazine attracts the leading edge of the sheets. The device has a slide which limits the effect of perforations depending on the size of the shingled sheet.
U.S. Pat. No. 4,268,025 (Murayoshi) describes a top sheet feeding apparatus wherein a sheet tray has a vacuum plate above the tray which has a suction hole in its bottom portion. A feed roll in the suction hole transports a sheet to a separating roll and a frictional member in contact with the separating roll.
U.S. Pat. No. 4,418,905 (Garavuso) shows a bottom vacuum corrugation feeding system.
U.S. Pat. No. 4,451,028 (Holmes et al.) discloses a top feed vacuum corrugation feeding system that employs front and back vacuum plenums.
U.S. Pat. Nos. 868,317 (Allen); 1,721,608 (Swart et al.); 1,867,038 (Uphan); 2,224,802 (Spiess); 3,041,067 (Fux et al.); 3,086,771 (Goin et al.); 3,770,266 (Wehr et al.); and 4,382,593 (Beran et al.); all disclose sheet feeders in which a blower appears to be angled at sheets.
U.S. Pat. Nos. 3,837,639 (Phillips) and 4,306,684 (Peterson) relate to the use of air nozzles to either separate or maintain sheet separation.
U.S. Pat. No. 3,171,647 (Bishop) describes a suction feed mechanism for cardboard and like blanks that employs a belt which is intermittently driven.
U.S. Pat. No. 3,260,520 (Sugden) is directed to a document handling apparatus that employs a vacuum feed system and a vacuum reverse feed belt adapted to separate doublets.
U.S. Pat. No. 3,614,089 (Van Auken) relates to an automatic document feeder that includes blowers to raise a document up against feed belts for forward transport. Stripper wheels are positioned below the feed belts and adapted to bear against the lower surface of the lowermost document and force it back into the document stack.
U.S. Pat. No. 4,699,369 (Zirilli) is directed to a top vacuum corrugation feeder that employs an air knife that includes trapezodial shaped fluffer jets to enhance high speed feeding of a variety of paper weights.
IBM Technical Disclosure Bulletin entitled "Document Feeder and Separator", Vol. 6, No. 2, page 32, 1963 discloses a perforated belt that has a vacuum applied through the perforations in the belt in order to lift documents from a stack for transport. The belt extends over the center of the document stack.
The above-mentioned disclosures are included herein by reference to the extent necessary to practice the present invention.
It will be appreciated that while vacuum feeders have the advantages of high reliability and of generating less paper debris than, for example, friction retard feeders, and hence minimizing a significant dirt source in the machine, the paper debris affects the reliability and the quality of performance of many subassemblies, in particular, the photoreceptor. Conversely, the air power consumption is extremely high, specifically when recognizing that motor-blower efficiency is typically low. High power consumption is a barrier toward using vacuum feeders in low and middle volume machines.